A magnetic random access memory (MRAM) is a memory device using a magnetic element having the magnetoresistive effect in a cell portion for storing data, and is attracting attention as a high-speed, large-capacity, nonvolatile next-generation memory device. The magnetoresistive effect is a phenomenon in which when a magnetic field is applied to a ferromagnetic material, the electrical resistance changes in accordance with the direction of magnetization in the ferromagnetic material. The device can be operated as a memory device (MRAM) by writing by using the magnetization direction in the ferromagnetic material, and reading in accordance with the magnitude of electrical resistance corresponding to the magnetization direction.
Recently, it has become possible to obtain a very high magnetoresistance ratio (MR ratio) of 100% or more by the tunnel magnetoresistive effect (TMR effect) in a ferromagnetic tunnel junction including a sandwiched structure in which a tunnel barrier made of MgO is inserted between two ferromagnetic layers made of CoFeB. By taking advantage of this, a large-capacity MRAM including a ferromagnetic tunnel junction (magnetic tunnel junction [MTJ]) element using the tunnel magnetoresistive effect is regarded as promising and attracting attention.
When using the MTJ element in the MRAM, one of the two ferromagnetic layers sandwiching the tunnel barrier is used as a reference layer by using a magnetization-fixed layer in which the magnetization direction is fixed to be invariable, and the other is used as a storage layer by using a magnetization free layer in which the magnetization direction readily reverses. Data can be stored by making a state in which the magnetization directions in the reference layer and storage layer are parallel and a state in which they are antiparallel correspond to “0” and “1” of binary numbers. The tunnel barrier resistance (barrier resistance) is lower and a tunnel current is larger when the magnetization directions are parallel than when they are antiparallel. The MR ratio is represented by “(resistance in antiparallel state—resistance in parallel state)/resistance in parallel state”.
Stored data is read by detecting a resistance change caused by the TMR effect. Accordingly, the magnetoresistance ratio (MR ratio) obtained by the TMR effect is preferably high when reading. In contrast, data is written to a large-capacity MRAM by supplying a write current to the MTJ element, and switching the magnetization direction in the storage layer by the spin-transfer torque method. When writing, therefore, the MR ratio is preferably low because the write voltage is suppressed if the magnetization directions are antiparallel and the barrier resistance is high. A write requires a larger current than a read, so the absolute write voltage is larger than the absolute read voltage.
From the foregoing, the MR ratio is preferably high at a low voltage for a read and low at a high voltage for a write, i.e., the dependence of the MR ratio on the bias voltage is preferably steep.